Accumulation period (integration time) in DiFX.

I wanted to have to force the DiFX correlator to put each accumulation period (AP) (= integration time) at the regular grid across the entire experiment. The benefits of such a time tag assignemnt is that if the array observed source A in scan #1, source B in scan #2, and again source A in scan #3, scans #1 and #3 can be glued together and the fringe search procedure can be applied to the glued scan, provided the gap between scans is not long enough to hit the decorrelation limit.

Unfortunately, by default the DiFX correlator in August 2010 did not make the gaps between the last uv-point of the previous scan and the first uv-point of the next scan commensurate to the AP lengths.

Adam Deller in his letters of 2010.08.10 provided detailed explanation:

For continuous opbservation the sampling time in seconds is usually 1/2B, where B is the IF width in Hertz.

As far as I understand, the correlator uses (or may use?) flag data to bypass time when the station is not on source. The flag data are rounded to an integer second.

I noticed that the integration time in experiment BC191A specified as 0.25 sec in the key file (coravg = 0.25) processed with DiFX 1.5-3. Was changed to 0.26 that made the gap between scans not commensurate to AP lengths.

Adam continues:

Am I right that if "tweakIntTime off", then the number of sub-integrations in one AP will be the same, except the last AP?

2010.08.10_21:18:35